|Rank|Player|Points|Points defending|Points won|New points|Withdrew due to|
|---|---|---|---|---|---|---|
|5|Juan Martín del Potro|5115|720|0|4395|right wrist surgery|
|6|Nikolay Davydenko|5145|360|0|4785|broken wrist|
|20|Radek Štěpánek|1705|90|0|1615|fatigue|
|23|Tommy Haas|1660|180|0|1480|right hip surgery|
|32|Gilles Simon|1395|90|0|1305|right knee injury|
|36|Ivo Karlović|1295|10|0|1285|right foot injury|
|10|Kim Clijsters|3890|0|0|3890|left foot injury|

Question: how many players withdrew due to surgery?
Answer: 2